BEIRUT — Syrian security forces responded to President Obama’s call for the departure of Syrian President Bashar al-Assad by opening fire on anti-government protesters Friday, killing at least 18 people.

With many towns and cities around the country under almost complete military lockdown, some protests were smaller than they have been in past weeks. In many places, demonstrations were restricted to a few hundred people marching in side streets and alleys that afforded quick escape routes.

But there were signs that the country’s still largely leaderless protest movement has been revitalized by the signals of international support, as thousands of people turned out in dozens of locations across the country, their spirits buoyed by the calls from the United States and the European Union for Assad to step down, activists said.

In the central Damascus neighborhood of Midan, one of the capital’s few protest flash points, security forces opened fire with live ammunition almost immediately against hundreds of people who swarmed out of a local mosque after Friday prayers, in what has become a ritual over 23 weeks of protests.

Though the response was quicker and fiercer than usual, many of the protesters did not scatter but stood their ground and hurled stones as the bullets flew, according to an eyewitness and a scene from the encounter posted on YouTube.

“People today were really determined to stand up to them. When we heard the gunfire, a lot of people remained standing,” said the witness, an activist who was speaking via Skype from Damascus. “It was as if someone told us, ‘We have your back.’ We felt safer than before; we don’t feel isolated, because we know the international community doesn’t want this regime.”

The concern now, he and other activists said, is that the increasingly isolated government will feel it has no choice but to crack down more harshly, to crush the protest movement and head off any further attempts to replace Assad. In many locations around the country, the activist said, “we’ve entered a new phase now because what the U.S. has done is tell [Assad] there’s no way out now except to fight.”

Reem Haddad, a government spokesman, said Assad was expected to deliver an address to the Syrian people in the coming days to update them on his reform program, which, she said, Obama appeared intent on sabotaging with his call for the president to step down. “It is strange that instead of offering help to the reform program, Obama is seeking to bring more violence into Syria,” Haddad said.

Though the shootings on Friday appeared to contravene Assad’s assertion in a telephone call Wednesday night to U.N. Secretary General Ban Ki-moon that the military offensive had ended, Haddad said there was no contradiction.

Those opening fire were not army soldiers but regular security forces, and those being fired on were not protesters but “armed gangs causing havoc and terror,” she said.

Though Assad has repeatedly promised reforms, none has yet been implemented five months into the revolt, and U.S. officials have dismissed those offered as insufficient to address the scale of the discontent that has swept the country. An uprising that began with modest demands for reform has escalated into an outright rebellion in which the chief demand chanted by some protesters Friday was for “the execution of the president.”

With the United States and Europe having given up on demanding reforms, attention is switching to what may replace Assad. The fractured and leaderless opposition’s lack of structure or organization was cited as a major concern by U.S. and other Western officials as they debated whether to explicitly call for Assad to go.

Opposition figures say the intensified international pressure on the regime has boosted efforts to present a coherent alternative.

On Sunday, a group of mostly exiled Syrians meeting in Istanbul is expected to announce the formation of a Syrian National Council to represent the opposition, said Yaser Tabbara, a Syrian American lawyer based in Chicago who is helping coordinate the effort.

“It’s the alternative the international community has been looking for, a body that can speak for the opposition,” he said.

Late Thursday, an umbrella group bringing together the dozens of local committees that have sprung up inside Syria to organize protests was announced in a statement posted on the Syrian Revolution Facebook page.

The eventual goal, Tabbara said, is to merge the two groups into a Transitional National Council that will mimic the one formed in Libya, now widely recognized as Libya’s official government.

In Brussels, the European Union on Friday approved new sanctions against the Syrian government and pledged further steps to squeeze Syria’s banking and petroleum industries. E.U. foreign affairs chief Catherine Ashton said member states were preparing for a possible embargo on Syrian petroleum and a suspension of technical assistance from the European Investment Bank. The proposals could be approved as early as next week.

“The European Union continues to aim at putting an end to the brutal repression and assisting the Syrian people to achieve their legitimate aspirations,” Ashton said in statement released by her office.

Western diplomats also are hoping to increase the pressure on Assad through a criminal investigation, though the presumed investigative body, the International Criminal Court, has not yet been given the authority to conduct such a probe. In a statement Friday, ICC chief prosecutor Luis Moreno-Ocampo said his office had no jurisdiction to investigate allegations of Syrian crimes against humanity. Permission for a formal probe must be granted by the U.N. Security Council, which includes member states opposed to tougher measures against Syria.

Staff writer Joby Warrick in Washington contributed to this report.

Liz Sly is the Post’s Beirut bureau chief. She has spent more than 15 years covering the Middle East, including the Iraq war. Other postings include Africa, China and Afghanistan.

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